You know, I don't think I've ever actually heard a caged bird sing? I've heard them peep or tweet, just not actually "singing". I do hear wild birds singing all the time, though.
Related: I just read Ken Starr's "Contempt" yesterday and he makes several references to how much he enjoys - particularly when he's hiking through the woods in Virginia.
Anyway; I thought that was an interesting little thing.
:)